The present invention is related to International Application No. PCT/US99/12772 filed Jun. 8, 1999 entitled xe2x80x9cPharmaceutical Product and Methods and Apparatus for Making Same.xe2x80x9d
The present invention relates to improvements in an apparatus for the manufacture of pharmaceutical products.
In the pharmaceutical industry, pharmaceutical products are typically embodied as tablets, caplets, test strips, capsules and the like. Such products, which include diagnostic products, include one or more xe2x80x9cunit dosage formsxe2x80x9d or xe2x80x9cunit diagnostic formsxe2x80x9d (collectively xe2x80x9cunit formsxe2x80x9d).
Each of the unit forms typically contains at least one pharmaceutically- or biologically-active ingredient (collectively xe2x80x9cactive ingredientxe2x80x9d) and, also, inert/inactive ingredients. Such active and inactive ingredients, typically available as powders, are suitably processed to create the unit forms.
In the above-referenced International Patent Application, which is incorporated herein by reference, applicant discloses an apparatus for manufacturing such unit forms. The apparatus utilizes an electrostatic deposition process whereby powder(s) containing active and/or inactive ingredients are deposited on a substrate at discrete locations thereby producing the unit forms. To provide context for the present invention, the deposition apparatus, its operation, and illustrative unit forms produced thereby are described below.
FIGS. 1-4 depict one embodiment of a unit form 6 produced by the electrostatic deposition apparatus. FIG. 1 depicts a plurality of such unit forms 6 arrayed on a strip 4. In the illustrated embodiment, strip 4 comprises a substrate 8 and a cover layer 10, each of which comprise a substantially planar, flexible film or sheet. In some embodiments, one of either substrate 8 or cover layer 10 include an array of semi-spherical bubbles, concavities or depressions (hereinafter xe2x80x9cbubblesxe2x80x9d) 12 that are advantageously uniformly arranged in columns and rows.
Unit form 6 comprises active ingredient 14, a portion of cover layer 10 defining bubble 12, and a region of substrate 8 within bonds 7. FIG. 2 (showing cover layer 10 partially xe2x80x9cpeeledxe2x80x9d back from substrate 8) and FIG. 3 (showing a cross section of a portion of strip 4) depict a deposit of dry active ingredient 14, in the form of a powder, disposed between substrate 8 and cover layer 10 within bubble 12. FIG. 3 and FIG. 4 (showing a top view of a unit form 6) depict substrate 8 and cover layer 10 attached to one another via bonds 7 that are near to and encircle bubble 12.
Deposition Apparatus
FIG. 5 depicts, via a high-level block diagram, deposition apparatus 1 suitable for making unit form 6. Apparatus 1 comprises platform 102 wherein unit forms 6 are produced. Platform 102 performs a variety of operations including the electrostatic deposition of dry powder on defined discrete regions of a substrate, materials handling, alignment operations, measurement operations and bonding operations.
Electrostatically-charged powder is delivered to platform 102 for deposition via powder feed apparatus 402. In some embodiments, platform 102 and/or powder feed apparatus 402 are isolated from the ambient environment by an environmental enclosure. In such environments, environmental controller EC provides temperature, pressure and humidity control for platform 102 and powder feed apparatus 402. Further description of platform 102 and powder feed apparatus 402 is provided later in this section.
Processor P and controller C control various electronic functions of apparatus 1, such as, for example, the application of voltage for the electrostatic deposition operation, the operation of powder feed apparatus 402, the operation of robots that are advantageously used in conjunction with platform 102, and dose measurement operations. To facilitate such control functions, memory M is accessible to processor P and controller C.
FIGS. 6 and 7 depict a top view and a front elevational view, respectively, of illustrative platform 102. In some embodiments, platform 102 comprises bench 214 that incorporates five processing stations that perform various operations used to produce the present product. Briefly, those processing stations include: storage station 220, which advantageously comprises three substations 220A, 220B and 220C for storing substrates and cover layers; alignment station 230 for assuring that the substrate and cover layer are properly adhered to a transport mechanism (e.g., robotic elements) that delivers them to other processing stations; deposition station 250 where powder is deposited on the substrate; dose measurement station 240 for measuring the amount of powder that is deposited on the substrate; and lamination station 260 where the cover layer is laminated to the substrate.
As depicted in FIG. 7, four supports 216 elevate bench 214 above a table or like surface. Additionally, supports 216 advantageously provide a frame or superstructure for optional side-mounted barriers 218, depicted in FIG. 6. The side-mounted barriers, in conjunction with a top barrier (not shown) and bench 214 define an environmental enclosure or chamber that isolates the region therein from the ambient environment under air or inert gas.
To facilitate the various processing operations, as well as materials handling between the processing stations, platform 102 advantageously includes a transport means. In the embodiment illustrated in FIG. 7, the transport means is a robotic system that includes first robotic transport element 270 and second robotic transport element 280 that are movable along first rail 290. First rail 290 functions as a guide/support for movement in one direction (e.g., along the x-axis). An additional rail (not shown) movably mounted on first rail 290 functions as a guide/support for movement in a direction orthogonal to but in the same plane (e.g., the y-axis) as first rail 290. Such rails collectively provide x-y motion. Drive means (not shown), such as x-y stepper motors, move robotic transport elements 270 and 280 along the rails.
Receiver 272 is attached to first robotic transport element 270 and xe2x80x9cbondingxe2x80x9d head 282 is attached to second robotic transport element 280. Receiver 272 is operable to retrieve at least the substrate from the substation where it is stored (i.e., 220A or 220B or 220C) and to move it to at least some of the various operational stations 230-260 for processing. Bonding head 282 is operable to join/seal the substrate and cover layer to one another to create the unit forms 6.
First and second robotic transport elements 270 and 280 have telescoping components under servo control (not shown) that provide movement along the z axis (i.e., normal to the x-y plane). Such z-axis movement allows receiver 272 and bonding head 282 to move xe2x80x9cdownwardlyxe2x80x9d toward a processing station to facilitate an operation, and xe2x80x9cupwardlyxe2x80x9d away from a processing station after the operation is completed.
Moreover, robotic transport elements 270 and 280 advantageously include xcex8 control components under servo control (not shown) that allow receiver 272 and bonding head 282 to be rotated in the x-y plane as may facilitate operations at a processing station. Compressed dry air or other gas is suitably provided to operate the robotic transport elements. Robotic transport elements 270 and 280 can be based, for example, on a Yaskawa Robot World Linear Motor Robot available from Yaskawa Electric Company of Japan.
As previously indicated, powder comprising an active ingredient is electrostatically deposited at discrete locations on substrate 8 at deposition station 250. In the illustrated embodiments, accomplishing such deposition requires that, among other things, substrate 8 is transported to deposition station 250 from some other location, and that an electrostatic charge is developed that causes the powder to electrostatically deposit on substrate 80. Such transport and charging operations are facilitated, at least in part, via receiver 272 and electrostatic chuck 302.
FIG. 8 depicts a view of first surface 304 of electrostatic chuck 302. Electrostatic chuck 302 comprises a layer 303 of dielectric material. The electrostatic chuck has a thickness of about 0.01 inches (0.25 mm), and, as such, is relatively flexible. Illustrative electrostatic chuck 302 has xe2x80x9cthrough holesxe2x80x9d ECH implemented as slots that are disposed at its periphery. First surface 304 further includes a plurality of powder collection zones CZ. In illustrative electrostatic chuck 302, collection zones CZ are advantageously organized in eight columns 306C1-C8, of twelve collection zones each for a total of ninety-six collection zones CZ. As will be described in further detail later in this specification, each collection zone CZ corresponds to a powder deposition location on the substrate (see substrate 8 in FIG. 1). Collection zones CZ are formed within electrostatic chuck 302 by an arrangement of dielectric and conductive regions, several embodiments of which are described later in this section in conjunction with FIGS. 10a-10c. 
FIG. 9 depicts a view of second surface 308 of electrostatic chuck 302. As depicted in more detail in FIGS. 10a-10c, collection zones CZ are formed via electrical contact pads 310. Such electrical contact pads 310 provide contact points for connection to a controlled voltage source.
Electrical contact pads 310 are electrically connected to selected other electrical contact pads via address electrodes 312. By virtue of such groups of selected electrical connections (e.g., the pads 310 within a given column 306C1-C8 of illustrative chuck 302 of FIG. 9 define an illustrative grouping), a first voltage can be applied to contact pads 310 in column 306C1, while a second voltage different from the first voltage can be applied to contact pads 310 in second column 306C2, and so forth varying the voltage applied to contact pads 310 on a column-by-column basis as desired. It will be understood that the application of such different voltages to such different columns results in depositing a different amount of powder at collection zones CZ in each of such columns. In other embodiments, address electrodes are arranged differently thereby creating electrical interconnects between differently-arranged groupings of contact pads 310. For the layout of contact pads 310 and address electrodes 312 depicted in FIG. 9, voltage need only be applied to a single contact pad 310 within a given column 306 to develop substantially the same electrostatic charge at each contact pad 310 within that column.
FIGS. 10a-10c depict several illustrative embodiments of structural arrangements suitable for forming collection zones CZ within an electrostatic chuck, such as electrostatic chuck 302. For clarity of illustration, the structure associated with only a single collection zone CZ of an electrostatic chuck is depicted in FIGS. 10a-10c. 
In a first embodiment depicted in FIG. 10a, a conductive material 314 is disposed through layer 303 of dielectric at each region designated to be a collection zone CZ. The conductive material overlays a portion of first surface 304 and second surface 308 of the electrostatic chuck. The portion of conductive material 314 overlying first surface 304 comprises a powder-attracting electrode 316A, while the portion of conductive material 314 overlying the second surface 308 comprises electrical contact pad 310A (which is one embodiment of electrical contact pad 310 previously mentioned). A shield electrode 318 (also termed a xe2x80x9cground electrodexe2x80x9d based on a preferred bias) is disposed within layer 303.
Applying a voltage to electrical contact pad 310A generates an electrostatic field at powder-attracting electrode 316A at collection zone CZ. As described later in this section, the electrostatic field attracts charged powder to the substrate 8 that engages first surface 304 of the electrostatic chuck. Additionally, the electrostatic field aids in holding substrate 8 flat against first surface 304. Tight adherence of the substrate 8 to the electrostatic chuck increases the reliability, consistency, etc., of powder deposition at the collection zones. A reduced pressure that is developed in receiver 272 to which the substrate 8 is exposed also assists in adhering the substrate to the electrostatic chuck.
FIG. 10b depicts a second illustrative embodiment where via hole V is formed at electrical contact pad 310B and powder-attracting electrode 316B. FIG. 10c depicts a third illustrative embodiment wherein an additional layer 305 of dielectric material separates powder-attracting electrode 316C from substrate 8. Electrical contact-pad 310C overlays second surface 308.
The electrostatic chuck provided by the configuration depicted in FIG. 10c can be termed a xe2x80x9cPad Indent Chuckxe2x80x9d which is useful, for example for powder depositions of less than about 2 mg, preferably less than about 100 xcexcg, per collection zone CZ (assuming, for example, a collection zone having a diameter within the range of 3-6 mm diameter). The electrostatic chuck provided by the configuration depicted in FIG. 10a can be termed a xe2x80x9cPad Forward Chuckxe2x80x9d which is useful, for example, for powder depositions of more than about 20 xcexcg per collection zone CZ (again assuming a collection zone of about 3-6 mm diameter). The Pad Forward Chuck is more useful than the Pad Indent Chuck for higher dose depositions.
As described further below, electrostatic chuck 302 is engaged to receiver 272 during at least some deposition-apparatus operations (e.g., during electrostatic deposition of powder on the substrate 8). FIG. 11 depicts underside 274 of receiver 272 with electrostatic chuck 302 adhered thereto. Electrostatic chuck 302 has alignment features 320, such as pins or holes, by which it is aligned to complementary holes or pins (not shown) in the receiver. Also depicted are alignment pins 276 that are received by complementary holes in bench 214 for aligning receiver 272 to various processing stations (e.g., deposition station 250). Height-adjustable vacuum cups 278 are advantageously used to attach an alignment frame (not shown), which can be used in conjunction with the substrate, to the receiver.
The powder deposition process proceeds via electronic control of electrostatic chuck 302. As previously described, the deposition apparatus 1 advantageously includes central processor P and controller C for performing calculations, control functions, etc. (see FIG. 5). Processor P receives performance input from multiple sources, including, for example, on-board sensors and historical data from dose measurement station 240, and uses such information to determine if operating parameters should be adjusted to keep powder deposition within specification. Such input includes, for example, data pertaining to the rate of powder flux into and through the deposition engine (made up of powder feed apparatus 402 and deposition station 250) and the degree to which powder is being evenly deposited at electrostatic chuck 302. The xe2x80x9con-receiverxe2x80x9d electronics described below, either alone or in conjunction with processor 401 and controller 403, provide a means for adjusting apparatus 1 during operation.
In embodiments in which processor P has primary responsibility for processing functions, a secondary processor (not shown) located in receiver 272 functions as a communications board that receives commands from processor P and relays such commands to an addressing board (not shown), also located in receiver 272. The addressing board then sends bias control signals (DC or AC signals) for controlling the voltage applied to electrical-contact pads 310. Depending upon the addressing scheme (e.g., the arrangement, if any, by which individual electrical-contact pads 310 are electrically interconnected via address electrodes 312), voltage is either regionally (e.g., by columns, rows, etc.) or individually applied.
The addressing board preferably has multiple channels of synchronized output (e.g., square wave or DC). The signals sent to the addressing board can be encoded, for example, with a pattern of square wave voltage pulses of varying magnitudes to identify a particular electrical-contact pad/powder-attracting electrode, or a group of such electrodes, together with the appropriate voltage to be applied thereto.
The bias control signals are sent via a high voltage board (not shown), which advantageously has multiple channels of high-voltage converters (transformers or HV DC-to-DC converters) for generating the voltages, such as 200 V or 2,500 V or 3,000 V (of either polarity), that energizes powder-attracting electrodes 310. The high voltage board is advantageously located in receiver 272 so that other systems are isolated therefrom.
In some embodiments, the xe2x80x9csecondaryxe2x80x9d on-receiver processor receives data directly from xe2x80x9cchargexe2x80x9d sensors (not shown) that are positioned on or adjacent to electrostatic chuck 302. Such sensors monitor the amount of powder being deposited. The on-receiver processor locally interprets and responds to data from such sensors by suitably adjusting the voltage applied to the electrical contact pads/powder-attracting electrodes.
Operation of the Deposition Apparatus
In operation, first robotic transport element 270 moves receiver 272 and electrostatic chuck 302 adhered thereto (see FIG. 11) to storage station 220. At station 220a, electrostatic chuck 302 engages a xe2x80x9cvirginxe2x80x9d substrate and, in some embodiments, also engages an alignment frame (not shown) that is joined to the substrate.
In one embodiment, after engagement, robotic transport element 270 moves receiver 272, electrostatic chuck 302, the substrate and frame to alignment station 230. At the alignment station, the substrate is brought into contact with a pad (e.g., urethane foam, etc.). Such contact advantageously smoothes the substrate against electrostatic chuck 302. After the substrate is smoothed against the substrate, a suction force is applied that holds the substrate against electrostatic chuck 302. Flattening and smoothing the deposition surface (i.e., the substrate) in such manner improves the consistency of the powder deposits thereon.
Robotic transport element 270 then moves engaged receiver 272, electrostatic chuck 302, the substrate and frame to dose measurement station 240. After aligning with a measurement apparatus 242 at station 240, the substrate is scanned via a measurement device and distances from a reference point to the substrate at each collection zone CZ (see FIGS. 8, 10a-10c and 11) are calculated and recorded to provide baseline data.
Robotic transport element 270 then moves engaged receiver 272, electrostatic chuck 302, the frame and virgin substrate to deposition station 250. At deposition station 250, the substrate abuts gasket 259 that frames deposition opening 258 (see FIG. 6). The powder deposition engine (see FIG. 13) is turned on and powder is electro-deposited through deposition opening 258 on the substrate at regions overlying the electrostatic chuck""s collection zones CZ.
At the completion of the powder-deposition operation, robotic transport element 270 returns the substrate, with its complement of discreetly deposited powder, to dose measurement station 240. At that station, the measurement device again scans the substrate to determine the distance between the reference point to the surface of each xe2x80x9cdepositxe2x80x9d of powder. From such distances, and the previously obtained baseline data, the amount (e.g., volume) of powder in each deposition is calculated. If the calculated amount is outside a desired range of a predetermined target amount, such information is displayed. An operator can then suitably adjust operating parameters to bring the process back into specification. In another embodiment, automatic feed back is provided to automatically adjust the process, as required. The xe2x80x9cout-of-specxe2x80x9d unit forms may be discarded.
Regarding dose measurement, either one or both of two optical measurement methods may be used: diffuse reflection and optical profilometry, both of which methods are known in the art.
The diffuse reflection method is based on reflecting or scattering a probe light beam, such as a laser beam, off of the powder surface in directions that are not parallel to the specular reflection direction. Applicants have discovered that measurements obtained based on diffuse reflection using non-absorbing radiation provide a strong correlation with the deposited amount of powder in a unit form, at least up to a certain amount. The limiting amount varies with the character of the powder and is believed to correspond to an amount of powder that prevents light penetration into lower layers.
Diffuse reflection in a non-absorbing region provides good accuracy in measuring dose deposition amounts ranging from 50-400 xcexcg, or even as high as 750 xcexcg to 1 mg, for a 3 or 7 mm deposition xe2x80x9cdot,xe2x80x9d depending on the characteristics of the powder. The diffuse reflection method can detect substantially less than a mono-layer of powder. If the deposit is more than a mono-layer, the probe light beam must partially penetrate the upper layers so that it can be affected by the reflection off of the lower layers to provide an accurate measurement. There tends, however, to be a practical limit (dependent upon the powder) to deposition thickness for it to exhibit xe2x80x9cLambertianxe2x80x9d characteristics required for measurement via diffuse reflection. Diffuse reflection is also a measure of the physical uniformity of the dose deposits at the above-listed ranges.
Optical profilometry is useful for obtaining dose measurements that are above the ranges that can be accurately measured by the diffuse reflection method. In optical profilometry, light is directed to the deposit and scattered therefrom at an angle that is indicative of the height of the deposit. That height is readily calculated by triangulation. The profilometer can be, for example, a confocal profilometer. A confocal profilometer suitable for use in conjunction with the present invention is available from Keyence (Keyence Corp., Japan, or Keyence Corporation of America, Woodcliff Lake, N.J.) as Model LT8105.
Continuing, second robotic transport element 280 picks up a cover layer and, advantageously, an alignment frame from storage station 220 and delivers them to lamination support block 502 (see FIG. 12) at lamination station 260. After measurements are completed at dose measurement station 240, first robotic transport element 270 delivers the substrate with the deposited powder to lamination station 260. First robotic transport element 270 places substrate 8 on cover layer 10 such that the deposits of powder 14 are properly aligned within the perimeter of the bubbles 12 in the cover layer 10 (see FIG. 12).
After first robotic transport element 270 moves away, second robotic transport element 280 returns and, by the operation of bonding head 282, attaches the substrate and cover layer together, forming a plurality of unit forms on a strip (see FIG. 1). In an automated system, the unit forms may be automatically transferred to a packaging station wherein out-of-specification unit forms are screened out and in-spec unit forms are appropriately packaged.
Apparatus 1 for electrostatic deposition provides a product containing a plurality of pharmaceutical or diagnostic unit forms, each comprising at least one pharmaceutically or diagnostic active ingredient that advantageously does not vary from a predetermined target amount by more than about 5%.
The deposition xe2x80x9cengine,xe2x80x9d which comprises deposition station 250 on platform 102 and powder feed apparatus 402, can be a source of a variety of operational problems. Such problems include, for example, powder compaction, non-uniform powder flux, powder loading difficulties, operating instabilities and powder size limitations, among others. While the powder feed apparatus that is disclosed in International Application No. PCT/US99/12772 (and described briefly below) has been designed to avoid many of such problems, room for improvement in that apparatus exists. Such improvement is a goal of the present invention. Before addressing such improvements, which are described later in this Specification in the xe2x80x9cSummaryxe2x80x9d and xe2x80x9cDetailed Descriptionxe2x80x9d sections, an embodiment of the existing powder feed apparatus is described.
The Deposition Engine
Illustrative powder feed apparatus 402 includes powder-delivery system 403, which charges the powder via a powder-charging system 416 and delivers it to powder distributor 418. The powder distributor delivers the charged powder to deposition station 250 for deposition on the substrate 8 (electrostatic chuck and receiver not shown for clarity of illustration) that abuts gasket 259 framing deposition opening 258. Powder that is not deposited on the substrate is drawn back by a pressure differential through powder-evacuation tubes 426 to powder trap 428. Gas exiting powder trap 428 is delivered to HEPA filter 430.
In the illustrated embodiment, powder-delivery system 403 comprises auger rotation motor 404, hopper 406, vibrator 408, auger 410, clean gas source 414 feeding modified venturi feeder valve 412, and powder-charging system 416, interrelated as shown. In some embodiments, feeder valve 412 feeds powder-charging system 416. With the exception of powder-charging system 416, illustrative powder delivery system 403 is disposed substantially within enclosure 432, which is depicted in phantom for clarity of illustration.
In the illustrated embodiment, the powder-charging system is realized as a tube, referred to hereinafter as powder-charging feed tube 416. It will be understood, however, that in other embodiments, arrangements for powder charging other than the illustrated tube may suitably be used.
In place of venturi 412, a gas source can be provided to propel powder through powder charging feed tube 416. In one embodiment, gas source 414 directs gas pressure towards the outlet of a mechanical device that feeds powder. The gas jet can be directed and adjusted to act to de-agglomerate powder at that outlet.
In an alternate embodiment (not depicted), the hopper and auger arrangement depicted in FIG. 13 can be replaced with a rotating drum that temporarily stores powder and delivers it to a movable belt. The movable belt then transports the powder to a means for removing the powder from the belt. An example of such a means is a thin, high velocity jet of gas that blows the powder into powder charging feed tube 416 or a conduit in communication therewith.
For electrostatic deposition, the powder must be charged. This function is accomplished, as described above, by the powder-charging system (e.g., powder-charging feed tube 416). Some farther details concerning powder charging is now provided.
In one embodiment, powder charging feed tube 416 is made of a material that imparts, by triboelectric charging, the appropriate charge to the powder as it transits the tube making periodic collisions with the sides thereof. As is known in the art, TEFLON(copyright), a perfluorinated polymer, can be used to impart a positive charge to the powder (where appropriate for the powder material) and Nylon (amide-based polymer) can be used to impart a negative charge.
In so charging the powder, the tube builds up charge which can, if not accommodated, discharge by arcing. Accordingly, a conductive wrap or coating is applied to the exterior of powder charging feed tube 416 and grounded. Tube 416 can be wrapped, for example, with aluminum or copper foil, or coated with a colloidal graphite product such as Aquadag(copyright), available from Acheson Colloids Co. of Port Huron, Mich. Alternatively, powder charging feed tube 416 can be coated with a composition comprising graphite or another conductive particle such as copper or aluminum, an adhesive polymer, and a carrier solvent, mixed in amounts that suitably preserves the xe2x80x9ctackinessxe2x80x9d of the adhesive polymer. An example of such a composition is 246 g trichloroethylene, 30 g polyisobutylene and 22.5 g of graphite powder.
The charge relieved by the grounding procedures outlined above can be monitored to provide a measure of powder flux through powder charging feed tube 416. This data is advantageously sent to processor P for analysis. As a result of such analysis, deposition operating parameters can be modified, as appropriate, to maintain an on-specification operation.
Another way to impart charge to the powder is by xe2x80x9cinductionxe2x80x9d charging. One way to implement induction charging is to incorporate an induction-charging region in powder charging feed tube 416. More particularly, at least a portion of powder charging feed tube 416 comprises a material such as a stainless steel, which is biased by one pole from a power supply, with the opposite pole grounded. With an appropriate bias, an electric field is created in the induction-charging region such that powder passing through it picks up a charge. The length of the induction-charging region can be adjusted as required to impart the desired amount of charge to the powder. In one embodiment, induction charging is used in conjunction with the tribocharging features described above.
In yet another embodiment, powder is charged by xe2x80x9ccorona charging,xe2x80x9d familiar to those skilled in the art. See, for example, J. A. Cross, xe2x80x9cElectrostatics: Principles, Problems and Applications,xe2x80x9d IOP Publishing Limited (1987), pp. 46-49.
As previously indicated, powder charging feed tube 416 feeds charged powder via powder distributor 418 into deposition station 250, which is enclosed by enclosure 252. In the illustrated embodiment, powder distributor 418 comprises rotating baffle 424 that depends from nozzle 422. Nozzle motor 420 drives the rotating baffle.
Powder moving towards substrate 8 passes through control grid 254. Control grid 254 is advantageously disposed a distance of about one-half to about 1.0 inch below collection zones CZ of the electrostatic chuck (not shown in FIG. 12), and is biased at about 500V per one-half inch of such distance at the polarity intended for the powder. Control grid 254 thus xe2x80x9ccollimatesxe2x80x9d the powder cloud thereby attracting powder having an opposite charge (to the charge on the control grid).
Control grid 254 can be, for example, a series of parallel electrical wires, such as can be formed from xe2x80x9cswitchbacksxe2x80x9d of one wire, or, alternatively, a grid of wires. Spacing between parallel sections of wire is advantageously within the range of about 5 to about 15 mm. The rate of powder cloud flux can be monitored by measuring light attenuation between light emitter 256 (e.g., a laser emitter) and light detector 257. This value can be transmitted to processor P.
It has been found that fluctuations occur in the gas/powder flow through the deposition engine described above. Such fluctuations negatively impact deposition performance. The fluctuations are due, at least in part, to:
(1) the non-axisymmetric geometry of some embodiments of rotating baffle 424 and deposition station 250;
(2) the pulsing manner in which powder is delivered by some embodiments of powder delivery system 403; and
(3) flow instabilities due to boundary layer separation and vortex shedding.
It will be appreciated that it is desirable to reduce such gas/powder flow fluctuations to improve the performance of the deposition apparatus.
In accordance with the illustrative embodiment of the present invention, flow fluctuations observed in the existing deposition apparatus are reduced using a flow diffuser. The flow diffuser, which replaces the powder distributor of the existing deposition apparatus, comprises a conduit having a cross-sectional area that increases in the direction of powder flow. The increase in cross section controllably slows the gas flow to a velocity wherein electrostatic forces dominate the motion of the powder transported via the gas.
In some embodiments, the diffuser includes one or more flow control features. A first flow-control feature comprises one or more appropriately-shaped annular slits through which gas is injected into a xe2x80x9cboundary layerxe2x80x9d near the wall of the diffuser. The injected gas has a greater momentum than the gas in the boundary layer. Such injected gas serves several purposes, as itemized below.
1. Reducing the tendency for boundary-layer separation.
2. Directing/shaping the xe2x80x9cpowder cloudxe2x80x9d (i.e., the powder-transporting gas) towards a central axis of the diffuser. Such shaping counteracts an existing tendency for charged particles to repel one another, which tendency would otherwise cause the powder to migrate away from the central axis of the diffuser.
3. Providing a xe2x80x9cgas-curtainxe2x80x9d effect that reduces the tendency for powder contained in the powder cloud to get stuck against the diffuser wall.
A second flow control feature comprises one or more annular slits, or a multiplicity of slots/holes that are disposed at appropriate locations around the circumference of the diffuser. Such openings are in fluid communication with a pressure-differential generating means. The pressure-differential generating means generates a pressure differential across the openings in the diffuser such that pressure on the exterior of the diffuser is less than the pressure in the interior of the diffuser. As such, a portion of the powder-transporting gas in the slow-moving boundary layer is removed. Removing such slower-moving gas contributes to a flattening of the velocity profile of the powder-laden gas in the diffuser. And, such velocity-profile flattening tends to stabilize the powder-laden gas flow by preventing flow separation or at least delaying its onset.
Thus, the diffuser, the flow control features, and other elements related to powder delivery to the deposition station advantageously reduce spatial and temporal variations in the velocity of the powder-laden gas. The resulting increase in the uniformity of the flow-field improves control over the deposition operation. Such improved control results in an improvement in the uniformity and precision (i.e., the variation in the amount of active ingredient from a target amount) of depositions.